Thin homogeneous plasticized polymeric membranes (2.multidot.5 .mu.m) incorporating lipophilized components (highly selective ionophores) as the chemically acting species have been widely used in developing optical sensors and detectors [1, 2]. These membranes have advantages such as ease of fabrication into various thickness and shapes, high selectivity, sensitivity and reversibility. At present, various polymeric membrane based optical sensors have been successfully developed for small mono- and divalent cations, using cation chromogenic ionophores (chromoionophore) [3-7] or pH indicator and neutral carrier pair ion-exchange systems [8-12]. Ion coextraction systems are employed for reversible anion sensing [13-15]. Recently it has been demonstrated that polymeric membranes incorporated with a specific quaternary ammonium cation and a pH indicator can efficiently extract rather hydrophilic macrosized polyanions such as heparin and DNA. An optical sensing system for the detection of these polyanions has been developed using thin membrane films [16], however, no such system has been developed for the sensing of polycations, such as protamine.
Protamine is a low molecular weight protein (the average MW=4,500) rich in the basic amino acid--arginine [17] and is a polycation at near neutral pH. The guanadinium groups of protamine can complex electrostatically with the sulfonate groups of heparin, and thus it has been used to reverse the anticoagulant effect of heparin near the end of most clinical procedures that use heparin for systematic anticoagulation [18]. Heparin is also used in chronic therapy regimes to prevent deep vein thrombosis and other coagulation disorders. However, excess heparin in a post-surgical or chronic therapy patient increases the danger of "bleeding out" due to slow clotting response. Protamine is known to interact with conventional protein reagents, such as the Folin-phenol reagent and Coomassie Brilliant Blue G-250, and can be measured via either the Lowry [19] or Bradford [20] methods. However, these methods are not specific for protamine. New methods for fast and accurate protamine detection are of great interest. A new dye method for protamine detection in blood or plasma was developed recently, using the competitive binding displacement mechanism between protamine and heparin-azura A dye complex [21]. Furthermore, protamine ion-selective electrodes were developed based on a special formulated PVC poly(vinyl chloride) membrane doped with anionic tetraphenylborates as protamine carriers [22-23].